I have customized a session to add a session form command linked to a field.
The session works on worktop but pressing the same button on webtop hangs the client's browser.
Aren't form commands supported on webtop ?

Yes, they are supported.
Refer to the section for the standards, deprecated functions etc -

Quote:

ERP LN 6.1 Programmers Guide

Webtop Programming Rules

The following sections describe the deprecated API's:
Deprecated low-level UI drawing API's
Baan Automation is deprecated
Other deprecated predefined variables / functions
The following sections describe other attention points:
Focus events
Functions that should be used with caution
Some notes on the Generic Browser Framework (GBF)